How to identify canker sores.

- Lesion characteristics and locations. / Stages of formation. / Pictures. / Outbreak frequency and incidence rates. / Normal healing characteristics.

Canker sore

Link to ulceration diagram.

Canker sore

Link to ulceration locations animation.

Canker sore / Aphthous ulcer identification.

Dentists distinguish canker sores (specifically "recurrent minor aphthous ulcers," the type of canker lesion that affects 80% of sufferers) from other types of mouth sores by way of:

  1. Their appearance.
  2. Their location and the type of tissue on which they've formed.
  3. The fact that they are recurrent (have been experienced repeatedly).


Patient history also plays an important role in identification and differentiation from other kinds of lesions Like Herpes. - A person's dentist will be interested in signs and symptoms that did, and specifically did not, take place. Both immediately preceding, and then throughout the stages of their ulcer's formation and ultimate healing process.

No medical testing (e.g. biopsy, culturing) is typically involved in making a diagnosis. All of the above findings, collectively, can be expected to provide ample evidence for identification on their own. The microscopic characteristics of canker sores are non-specific.

What do canker sores look like? - The stages.

Note: The time frames we've assigned to the different stages outlined below overlap, due to the fact that what is experienced by any one person varies.

Stage 1 - (2 to 48 hours duration)

The earliest indication of pending canker sore formation doesn't involve visible signs.

Instead what is noticed is a sensation of tingling, itching, burning or numbness in the area where the lesion will ultimately form. (These signs are referred to as "prodromal" symptoms.)

Stage 2 - (18 to 72 hours duration)

During this time frame, the first visible signs of the developing lesion will have started to show.

  • The area where the sore will form begins to transform into a reddened circular area on the skin. (Formally referred to as an "erythematous macule.")
  • As the lesion's development progresses, the macule may transform into a small, possibly whitish, papule (solid, slightly raised area).
  • During this stage, the area will begin to ulcerate and enlarge. (The process of ulceration begins in the skin's basal layer and then advances into the more superficial layers above.)


A slideshow of pictures of aphthous ulcers.

Pictures of canker sores (aphthous ulcers).

Stage 3 - (1 to 14 days duration)

By this stage in its development, the lesion has transformed into a fully-formed ulceration (involves multiple layers of skin tissue). It may continue to grow in size for 4 to 6 days.

Appearance (of the typical fully-formed canker sore).

The physical characteristics of a canker sore.

A diagram showing the physical characteristics of an aphthous ulcer.

Sores have a grayish membrane-coated central ulceration with a surrounding red border. The skin around the lesion looks normal.

  • The lesion's center (the ulceration) develops a loosely attached membrane-like covering (a pseudomembrane) that can be easily dislodged.

    (The membrane is pyogenic in nature, meaning it's composed of cells, compounds and debris originating from the inflammatory process that has caused the ulceration.)

  • Initially, the membrane has a primarily yellow-white coloration. As healing ultimately progresses, it will tend to take on more of a grey-white tint.
  • A border comprising a band of erythematous (reddish) tissue will surround the ulceration.

    The outline of this red halo is characteristically circumscribed (well defined) and regular (not jagged).

  • The skin that lies outside the lesion's reddened border will appear normal and healthy.


Other characteristic signs and symptoms.

  • Canker sores usually are painful. The level of discomfort that's experienced may seem out of proportion with the lesion's size.

    Sufferers will frequently limit their oral movements, the types of foods they eat and favor the sore's surface so not to trigger pain.

  • There are no distinguishing systemic features associated with minor aphthous ulcers, such as the person having malaise (generally feeling poorly) or fever.

Section references - Neville, Akintoye, Chavan, Casiglia, Scully, Dunlap

Can a person have more than one lesion at a time?

Yes, outbreaks of multiple, simultaneous canker sores may occur.

  • Usually, the maximum number of sores that will form at one time will be six or fewer.
  • If multiple canker sores do develop, they're more likely to be distributed as opposed to being clustered together.
  • In the case where two sores do form adjacent to each other, they may combine into a single larger and irregularly shaped ulcer (as opposed to the classic round or oval shape).

Section references - Neville

Are they contagious?

  • No, canker sores are neither contagious nor infectious.


Where do canker sores form?

They characteristically only form on the "loose" (movable) tissues of the mouth, meaning those areas where the skin is not tightly bound to the bone underneath. These locations generally involve the "nonkeratinized" (softer, less tough) tissues of the mouth.

Location, location, location.

An important aspect of canker sore identification is based on the fact that they characteristically only form in certain parts of the mouth. These locations are, listed here in order of locations/tissues most affected:

Where canker sores usually form.

Animated graphic showing the mouth locations where aphthous ulcers typically form.

The ulcerations form on the "loose" tissues of the mouth.

  • The inside surface of the lips and cheeks.
  • The underside (ventral surface) of the tongue.
  • The mucobuccal fold (the deep trough where the skin of the jawbones and lips/cheeks meet).
  • The floor of the mouth.
  • The soft palate.
  • The tonsillar areas.

(Listed in order of locations/tissues most affected.)

Less common locations.

While rare, it is possible for canker sores to form on keratinized tissues. This includes the top (dorsal) surface of the tongue, the gum tissue that surrounds a person's teeth, hard palate, or even the border of the lips (vermelion border).

When this occurs, the lesion typically involves an extension of a sore already formed on adjacent nonkeratinized tissues.

Section references - Neville, Dunlap

Canker sore healing.

Healing time frames.

Canker sore (recurrent minor aphthous ulcers) characteristically heal within a certain time frame.

Therefore, what is experienced can be used to confirm lesion identification.

  • A sore will usually heal within 4 to 14 days, although some can heal in as little as 3 to 5.

    The pain associated with the lesion usually begins to subside around day 3 to 4.

  • In most cases, healing is uneventful and results in no residual scarring.
  • Any ulcer that has not healed within a 2-week time frame should be evaluated by a dentist.

    (Beyond just lesion differentiation, there is another larger kind of aphthous ulcer that characteristically does have an extended healing time frame.)


Healing appearance.

  • New skin growth (epithelialization) will begin at the edges of the ulceration and then close in from all directions as the healing process progresses.
  • The original yellow-white coloration of the pseudomembrane that covers the ulceration's surface will transform into a more grey-white appearance as the epithelialization process advances.

    Of course, this covering will be gradually replaced at its edges by new pink skin as the size of the lesion shrinks.

  • The reddened border of the original wound will remain obvious during the early stages of the healing process. Then, as the new skin tissue matures, the border's redness will fade. Complete healing and a normal appearance will return to the affected area within the time frame stated above.

Section references - Woo, Casiglia, Scully

How often do canker sores form?

Another important aspect of canker sore identification involves correlating characteristics associated with their occurrence.

Outbreak frequency.

  • Once a person has experienced an initial outbreak, the probability of recurrence is high, although the frequency with which this takes place can be quite variable.
  • A rate of one outbreak every 1 to 3 months might be considered typical (encompassing about 50% of people who get them). 30% of sufferers deal with their presence on a monthly basis.

    That means many people will only be plagued by a few episodes a year, while others will experience nearly continuous outbreaks and therefore will never be completely free of ulcers for any extended period of time.


Incidence rates / Prevalence.

  • Most sources tend to suggest that around 20% of the general population experiences canker sores, although actual reports from studies have ranged from 5% to 66%.
  • A person's first outbreak will typically take place between the ages of 10 and 20 years, and then decrease in frequency and severity with age.

    Lesions are most prevalent in people ages 10 to 40. 80% of those affected will experience their first lesion prior to the age of 30.

  • It's been suggested that women are more likely to experience sores than men but not all studies confirm this fact.


  • A family (genetic Details) predisposition seems to exist. Children with parents who get canker sores have a 90% chance of getting them too, as compared to a 20% chance for those whose parents don't.
  • A person's risk seems to be unrelated to race. But people who live in North America are more likely to suffer from them, as opposed to people who live in other world-wide geographic areas.
  • Non-smokers and people in upper socioeconomic groups are more likely to experience outbreaks.

Section references - Akintoye, Woo, Ship, Ship, Chavan, Neville

What are "major aphthous ulcers"?

Canker sores like we have described above are formally termed "recurrent minor aphthous ulcers." They are the type of canker lesion experienced by 80% of sufferers.

Beyond these "minor" lesions, there's another type of aphthous ulcer referred to as "major aphthae" or Sutton's ulcers. And in comparison to the minor variety, they are large, deep ulcerations whose healing is characteristically slower and more painful.

  • Unlike minor aphthae, the major kind forms on all types of oral tissues (both keratinized and nonkeratinized tissues). They frequently form on the lips, soft palate or fauces of the throat.
  • They can approach 1/2 inch (or more) in diameter and may cause regional or even facial swelling.
  • Their healing usually takes between 10 and 40 days. However, it may take some months (even as new ulcers are forming).
  • Healing is often associated with scarring.

Section references - Chavan, Neville


 Page references sources: 

Akintoye SO, et al. Recurrent aphthous stomatitis.

Casiglia J. Recurrent aphthous stomatitis: Etiology, diagnosis, and treatment.

Chavan M, et al. Recurrent aphthous stomatitis: a review.

Dunlap CL, et al. A guide to common oral lesions.

Neville BW, et al. Oral and Maxillofacial Pathology. Chapter: Allergies and Immunologic Diseases.

Scully C, et al. Oral medicine — Update for the dental practitioner Aphthous and other common ulcers.

Ship J, et al. Recurrent Aphthous Stomatitis.

Ship II. Epidemiologic aspects of recurrent aphthous ulcerations.

Woo S, et al. Recurrent Aphthous Ulcers: A Review of Diagnosis and Treatment.

All reference sources for topic Canker Sores.